1. Field of the Invention
The present invention relates to an apparatus and a method for detecting the concentration of NOX contained in a gas to be detected (hereinafter referred to as an “object gas”).
2. Description of the Related Art
A detection element has been known whose output changes with the concentration of a specific gas (oxygen, NOX, etc.) contained in exhaust gas discharged from an internal combustion engine such as an automotive engine. For example, a detection element capable of detecting the concentration of NOX has a layered structure including at least one pump cell composed of a solid electrolyte member and a pair of electrodes provided thereon. The detection element includes a first measurement chamber (first internal cavity) into which exhaust gas is introduced via a diffusion resistor section (first diffusion path); and a second measurement chamber (second internal cavity) into which exhaust gas, whose oxygen has been pumped out in the first measurement chamber, is introduced (for example, see Non-patent Document 1).
Oxygen contained in the exhaust gas introduced into the first measurement chamber is pumped out of the first measurement chamber to the outside by a first pump cell, whereby the concentration of oxygen remaining in the exhaust gas introduced into the second measurement chamber is adjusted to a predetermined low level. NOX contained in exhaust gas includes NO and NO2. According to Non-patent Document 1, most of the NO2 is reduced to NO in the first measurement chamber. In the second measurement chamber, NO is decomposed to nitrogen and oxygen by the catalytic action of an electrode formed of noble metal such as Pt or Rh. At that time, oxygen derived from the decomposed NO (oxygen as a constituent of NO or NO2 (NOX)) is pumped out by a second pump cell. In the second pump cell, electrons conveyed via oxygen ions are detected in the form of a current. The oxygen concentration detected in this manner is offset by the concentration of residual oxygen (the above-mentioned adjusted oxygen concentration), whereby the concentration of NOX-originating oxygen (ultimately, the concentration of NOX) is detected.
[Non-patent Document 1] N. Kato et al., “Thick Film ZrO2 NOX Sensor”, SAE Technical paper series 960334 (1996)
3. Problems to be Solved by the Invention
However, the detection element merely detects the concentration of NOX; i.e., the mixture of NO and NO2, and cannot detect the concentration of NO and the concentration of NO2 individually.